In adhesion of base materials such as rubbers, leathers, metals, plastics such as polyvinyl chloride (PVC), foamed materials or fibers with the use of aqueous polyurethane solutions or dispersions, it is generally considered that the tack-fee time (open time) of the adhesives is a very important factor in addition to the adhesive strength and the durability.
To bond base materials on which an adhesive in the form of an aqueous dispersion is applied, it is required to employ a so-called reactivation step of drying at, for example, about 50 to 60° C. after applying the adhesive to the base materials so as to remove the moisture and achieve the adhesive performance. In case where the time taken from the reactivation step to the bonding of the base materials having the adhesive applied thereon is too long, there arises a problem that the adhesive surface loses its tackiness and thus achieves only insufficient adhesive performance. The tack-free time (open time) of an adhesive is largely affected by the environmental temperature and the surface temperature of base materials in the application and bonding steps. In case where the environmental temperature and the surface temperature of the base materials are low, the tack-free time (open time) is liable to be shortened. At higher temperatures, on the contrary, the tack-free time (open time) is liable to be prolonged.
Accordingly, it has been strongly required to provide adhesives which are little affected by the environmental temperature and the base material surface temperature in bonding various materials such as shoes and rubber shoes, have a long tack-free time (open time) allowing to take a sufficient time from reactivation to bonding, and yet are excellent in adhesive strength and durability.
Methods of producing aqueous dispersions of polyurethane resins are described in U.S. Pat. Nos.3,036,998, 3,756,992, etc.
Further, JP-B-59-30186 (The term “JP-B” as used herein means an “examined Japanese patent publication”) discloses polyurethane resins obtained from amorphous polyester polyols having metal sulfonate group with polyisocyanate compounds. It is described in this document that when applied on polyethylene terephthalate films, the polyurethane resins produced by the above method showed improved adhesiveness, water resistance, etc.
Although polyester polyols having metal sulfonate group are used in the adhesives comprising the aqueous dispersions of the polyurethane resins as described above, this component mainly comprises aromatic polyester polyols. Thus, these adhesives have a high reactivation temperature of 100° C. or above and the adhesion should be carried out under heating to a high temperature. In case of using in adhering or bonding general base materials such as shoes and rubber, therefore, the adhesives have a short tack-free time (open time) and suffer from a fear of damaging the base materials by heat, thereby failing to achieve any sufficient performance.
Japanese Patent No. 2,894,494 discloses a process for producing aqueous solutions or dispersions of polyisocyanate polyaddition compounds having carboxylate and/or sulfonate as hydrophilic group(s). It is described in this patent that reactivation of these adhesives applied to base materials can be carried out by treating at a low temperature. However, these adhesives suffer from a problem of having a short tack-free time (open time), which is an important factor in application properties of adhesives, and only unsatisfactory adhesive performance such as adhesive strength.